1. Field of the invention
The present invention relates to a transmitting device which switches the frequency of a digital transmission signal to a different frequency equivalent to the level thereof to thereby generate a frequency modulation signal and outputs the digital transmission signal in the form of the thus generated frequency modulation signal.
2. Related art
For example, when driving a load or the like carried on a car by remote control, a transmitting device for operation is used to transmit a remote control signal for the car in the form of an electric wave signal. The electric wave signal can be generated, for example, by frequency modulating a digital transmission signal corresponding to an identification code which is inherently set for the car. When a receiving device provided on the body side of the car receives the electric wave signal, then the electric wave signal is demodulated to the identification code. After then, if the identification code received is decoded and is found identical with the identification code set in the car, then the load corresponding to the identification code is driven or controlled by a drive device or the like.
Conventionally, as a transmitting device of this type, there is known a structure which is shown in FIG. 3. In FIG. 3, a CPU 1 can be operated in accordance with a clock signal for operation which is given by an oscillation circuit 2 for clock. That is, if a transmission switch (not shown) is turned on to thereby instruct the start of transmission, then the CPU 1 reads out an identification code consisting of a combination of "1", "0" digital signals from a memory 3 in which identification codes are stored and, in accordance with the levels of "1", "0" of the respective bits, outputs a control signal to an oscillation circuit 4 for outputting a frequency modulation signal. The frequency modulation signal outputting oscillation circuit 4 is arranged such that, in accordance with the control signal, it switches the oscillation frequency to one of different frequencies f1 and f2 and then outputs a frequency modulation signal equivalent to the identification code. That is, in this manner, the frequency modulation signal equivalent to the identification code is generated and is then output as the electric wave signal.
However, in the above-mentioned conventional transmitting device, the frequency modulation signal outputting oscillation circuit 4 is indispensable to the device, which sets a limit to the size reduction of a transmitting device such as an electric wave key or the like.